1. Field of Invention
The present invention relates to using a bob with off-centered axial direction holes for testing the rheology of non-Newtonian fluids and overcoming the Weissenberg effect.
2. Description of Prior Art
The process of drilling wells, particularly in the petrochemical industry, has changed dramatically over the past few years, with much of that change having to do with the chemical makeup and rheological behavior of the fluids used to manage and operate the wells. The drilling process has become much less dependent on brute-force mechanical methods and turned instead to using chemistry to accomplish many of the necessary functions of drilling the well. This has necessitated a much greater understanding of the predicted viscoelastic behavior of chemical fluids, particularly those under down-hole conditions which can be extreme and changeable in ways that are difficult to anticipate.
Testing mechanisms for these fluids have also changed significantly, as the viscosity, temperature sensitivity, and rheological behavior of the tested fluids has diversified. This improved type of test equipment must be capable of measuring wider ranges of test fluid properties than in the past, with faster and more repeatable measurement response, while remaining as reliable, accurate, durable and easy to use as possible.
Another challenge pertains to testing non-Newtonian fluids, as these fluids commonly exhibit the “Weissenberg effect”. The Weissenberg effect describes the tendency of a non-Newtonian fluid to climb up a rotating rod. If there is air or pressurization gas above the sample fluid, the Weissenberg effect also causes the sample fluid to climb out of the measurement range of a co-axial cylinder geometry rheometer, thus causing inaccurate measurement. Therefore, it would be desirable to provide a method for determining the rheology of non-Newtonian fluids without the detrimental impact of the Weissenberg effect.
U.S. Pat. No. 6,412,338 teaches a viscometer containing a bob with circumferentially spaced openings, but the design of the bob is intended for a measurement chamber filled with one type of homogenous fluid only. If there is gas above the sample fluid in the measurement chamber, a non-Newtonian sample would still climb out of the measurement gap between the coaxial bob and cup and thus the design provides no remedy to the problem of the Weissenberg effect. U.S. Pat. No. 6,412,338 would also suffer from measurement inaccuracy because the radial direction holes on the bob surface would cause non-uniform shear rates at the location of these holes and thus provide inaccurate results.
Therefore, it is an object of this invention to provide a bob configured for the rheological testing of non-Newtonian sample fluids with gas or empty space on top of tested sample fluid while overcoming the Weissenberg effect in any test environment.